1. Field of the Invention
The invention relates to a traction drive having infinitely variable ratio characteristics, the traction drive including torque-transmitting discs forming a torque flow path between a driving shaft and a driven shaft.
2. Background Art
In a transmission mechanism with continuously variable torque ratio characteristics, the overall torque ratio may be changed without the usual shift between defined torque ratios. This will provide a lower engine speed for a given power output when the transmission mechanism is used in an engine-driven automotive vehicle driveline. Throttle losses are reduced and the engine operates close to its minimum brake specific fuel consumption point.
It is known design practice to achieve a relatively high torque-transmitting capacity in a continuously variable transmission environment by using toroidal discs as part of a traction drive. Torque-transmitting rollers are in engagement with the toroidal discs in a lubricated contact patch. The torque capacity that can be achieved exceeds the usual torque capacity of other well known continuously variable transmissions, such as belt and pulley assemblies wherein the radius of the contact of the belt with each pulley changes as the effective pitch diameter of the pulley is adjusted.
In the case of a toroidal traction drive, torque transfer through the rollers between the discs occurs through the lubricated contact patch by relying on shear forces at the interface of the rollers and the discs. An example of a toroidal drive of this kind can be seen by referring to U.S. Pat. No. 5,052,236, which discloses a traction drive in which two pairs of discs are used in a parallel torque flow arrangement, thereby increasing the torque-transmitting capacity by a factor of two compared to a traction drive with one pair of discs. Another example of an earlier traction drive design may be seen by referring to copending U.S. patent application Ser. No. 09/753,696, filed Jan. 3, 2001, entitled xe2x80x9cControl System for a Continuously Variable Traction Drivexe2x80x9d, Attorney Docket No. FMC 1120 PUS. This copending patent application is assigned to the assignee of the present invention.
In the traction drives disclosed in the copending application and in the ""236 patent, the transmission ratio is controlled by adjusting the rotary axis of each roller relative to the rotary axis of the discs. This creates a misalignment of the tangential force vectors from the rolling elements across the contact patch and produces a force vector component of the tangential force, which creates a roller tilting moment. This tilting moment adjusts the angularity of the rollers to change the transmission ratio.
When torque is transmitted through a traction drive transmission, control pistons of a hydraulic ratio control system provide a reaction force to the rollers to maintain their position. Reaction forces for these prior designs are relatively high, which requires a control effort that is difficult to achieve. In a typical automotive vehicle driveline using such a traction drive, the reaction force that must be accommodated by the hydraulic control system can easily exceed 5000N/roller. This high force creates a design challenge because of the sensitivity of the relationship between transmission ratio and the hydraulic pressure acting on hydraulic control pistons.
In the prior traction drive designs, power is transferred from an input disc through the rollers to an output disc across a traction fluid medium in a contact patch. The discs rotate in opposite directions about a common axis. The rollers are supported by trunnions on a trunnion frame. The transmission ratio is changed as the effective torque input radius and the effective torque output radius at the contact patches for the discs are changed due to an adjustment of the roller orientation created by laterally offsetting the roller axis relative to the disc axis. A steering moment created by the offset is generated to force the rollers to tilt to a new ratio. When that ratio is achieved, the offset is eliminated and the force component creating a tilting moment returns to zero. The new position of the rollers must be held accurately, however, so that the tilting moments on the rollers do not exceed the friction in the trunnion bearings. Furthermore, the high hydraulic control forces required make it difficult to package the hydraulic control elements in an assembly that would be practical for use in a commercial driveline. For this reason, traction drives typically are limited to two rollers for each pair of discs. This essentially limits the torque capacity of the driveline.
It is an objective of the invention to provide a traction drive in which ratio changes in a ratio variator can be achieved without displacing the axis of the rollers laterally relative to the axis of the discs. This is accomplished by angularly offsetting the roller trunnions about a gimbal axis parallel to the disc axis, which creates a steering moment that adjusts the inclination of the rollers. This isolates the control effort for controlling the inclination of the rollers about the gimbal axis from the load-bearing requirements that are typical of the traction control of the prior designs. It also substantially reduces the control effort relative to the control effort typical of the prior designs. Further, a so-called master roller may be controlled while driving a number of slave rollers through steering synchronization mechanisms. In this way, the rollers can readily be packaged in a group of three within each toroidal cavity created by the pairs of discs. This results in a 50% increase in torque capacity with little weight penalty, if any, compared to a design with one roller pair.
It is an additional objective to eliminate the need for dealing with the high gimbal control forces typical of the prior designs, thereby increasing the stability of the system and improving the response of the traction drive to the ratio control effort.
It is an objective also to support the trunnion by a gimbal frame whereby the torque reaction forces are transmitted to a gimbal frame rather than to a yoke and piston mechanism typical of the prior designs. The frame can be inclined about a pivot axis that passes through two fixed gimbal supports.
The trunnion is adjusted by a control cam or by other suitable control elements in a direction transverse to the direction of the disc axis. This inclines the gimbal frame, thereby producing an angular offset of the rollers. This offset generates a moment to produce a change in ratio. Thus, control forces are essentially decoupled from the force that is required to support the rollers.
It is a further objective to provide a traction control system having a single control mechanism associated with a so-called master roller and to use a number of slave rollers with a steering synchronization mechanism. This makes it possible to package three rollers between each pair of discs, as mentioned above, to improve the torque-transmitting capacity of the traction drive. The tilting of one roller is accompanied by corresponding tilting of the companion rollers because of the action of the steering synchronization mechanism.